The present invention relates generally to remote distance measurement by means of a received signal in a noise modulated signal transceiver. Specifically, the invention relates to remote distance measurement of signal reflecting objects in relation to a signal transceiver by means of the method and signal transceiver according to the preambles of claims 1 and 19 respectively. The invention also relates to a digital group antenna system.
Technical solutions where noise modulated radar transceivers are employed to determine distances have been known for a long time. Compared with other modulating principles the advantage of noise modulation is that a high distance resolution may thereby be obtained. Moreover, it is possible to transmit a radar signal at a high average power level, which in turn further enhances the radar's performance. A common application is binary phase modulation with a limited code length. However, there are also other types of noise modulation, for example in respect of the amplitude of a carrier wave. Noise modulated radar transceivers are likewise well suited for military applications, since it is comparatively difficult to reveal the noise signal. Provided that a high power level is utilized and a high resolution doppler filtering is employed, noise modulated radar devices show a very good immunity to noise disturbances.
Conventional pulse radar technology normally causes a problem with respect to ambiguity in distance. The problem is a consequence of that it must be possible to relate each radar echo to a particular transmitted pulse, such that a delay relative this pulse can be established. The delay in turn determines the distance to the object which produced the radar echo. A common method to mitigate the effects of the ambiguity problem is to use a variable pulse repetition frequency (PRF), so-called staggered PRF, whereby the time interval between two consecutively transmitted radar pulses varies from pulse to pulse. The ambiguity in distance is then resolved by registering multiple reflected pulses from one and the same reflecting object.
Noise modulated radar technology, however, requires no such methods, instead a reliable distance measurement may be accomplished on basis of a single radar pulse. Namely theoretically, the non-repetitive properties of the noise renders the radar's unambiguity interval infinitely long. Practically of course, the range is limited by other factors, such as the transmitter's output power level, the receiver's sensitivity and a longest acceptable delay.
FIG. 1 shows a block diagram over a per se known noise modulated radar transceiver 100. The following description of remote distance measurement with reference to FIG. 1 will pertain to a radar application, where a transmitted probing pulse is constituted by a radar signal, which propagates at the speed of light in the transmission medium in question. The remote distance measurement principle is, however, applicable also to other types of probing signals, such as sound waves. Naturally, these waves have a propagation speed which is considerably lower than for corresponding radar waves in all transmission media. Nevertheless, otherwise the conditions are in principle the same as in the radar case. Sound waves are advantageous as probing signals within many areas where radar signals are less suitable, for example in military- and civil sonar-/sodar-applications (for fish sweeping, mine sweeping and submarine sweeping) and in medical applications.
A noise generator 101 generates a noise signal x(t), which on one hand is fed to a delay element 103 in the form of a leak signal, and on the other hand is transmitted via a transmitter antenna 102 towards an object 104. The object 104 reflects a part of the radar signal x(t) against the radar transceiver 100, where a reflected radar signal x(t−T) is received via a receiver antenna 105. The reflected radar signal x(t−T) thus constitutes a delayed version of the transmitted radar signal x(t), where the delay is proportional to the distance R between the radar transceiver 100 and the object 104. A mixer 106 brings together the reflected radar signal x(t−T) and a reference signal x(t−Tf), which has passed through the delay element 103. The mixer 106 delivers a resulting signal to a lowpass filter 107. Given that the lowpass filter 107 has an impulse response h(t) the output signal Y(t) from the filter 107 may be described by the integral:Y(t)=∫x(t−τ−T)x(t−τ−Td)h(τ)dτwhere Tf denotes the delay of the transmitted radar signal x(t), which the delay element 103 generates. The reflected radar signal's x(t−T) delay T in relation to the transmitted radar signal x(t) is T=2R/c, where R=the distance between the radar transceiver 100 and the object 104 and c=the radar signal's propagation speed, which is equal to the speed of light in the transmission medium in question (for example vacuum, air, water or soil layer).
When the delay T of the reflected radar signal x(t−T) corresponds to the delay Tf of the delay element 103 the mixer 106 generates a powerful correlation peak, which may be used for determining the distance R to the object 104. Provided that a so-called distance slot is established at a distance R0=cTf/2 and the object moves through this distance slot R0 (due to the fact that at least one of the object 104 and the transceiver 100 moves) the output signal from the mixer 106 describes a high resolution, and in relation to the radar transceiver 100 a radial, reproduction of the object 104. The distance resolution ΔR for this reproduction is given by the relationship ΔR=c/(2B), where B=the bandwidth of the noise utilized. Thus, the larger the bandwidth B, the better the distance resolution ΔR becomes.
Given that the noise has an autocorrelation function RX(T−Tf) and that the lowpass filter 107 has a cut-off frequency, which lies considerably below the bandwidth of the noise B the output signal Y(t) has an average value <Y(t)>=RX(T−Tf). The filtering in the lowpass filter 107 can thereby be regarded as a form of an integration/averaging. When the product between the band-width of the noise B and the integration time Tint, i.e. the time during which data to the output signal Y(t) is collected, attains high values the output signal Y(t) approaches its average value <Y(t)>. Hence, the following approximation can be made:Y(t)≈∫Rx(T−Tf)dτ=TintRX(T−Tf)
From this expression it is then possible to determine the delay T of the reflected radar signal x(t−T) by performing a search by means of the delay element's 103 delay Tf after the maximum value of the output signal Y(t). When this value of the delay element's delay Tf has been determined, the distance R to the object 104 may also be calculated according to R=cT/2.
However, the search for the maximum value of the output signal Y(t) involves an operation that is relatively intense in terms of calculation complexity, which either causes a comparatively long delay or demands a very powerful hardware. The solution becomes particularly processing intensive if also the relative velocity of the object 104 should be determined with a high accuracy.